1. Technical Field
The present invention generally relates to data communications and particularly relates to upstream power back off and multiple user detection in communications schemes such as Very High Speed Digital Subscriber Loop (VDSL) communications.
2. Description of Related Art
VDSL communications technology is emerging as the DSL technology for two-way broad band network access. It is expected to be primarily deployed within a “fiber-to-the-cabinet” architecture. VDSL standards are in an advanced specification phase at regional standards bodies. Work is also underway at International Telecommunication Union—Telecommunication Standardization Sector (ITU-T) to define an international VDSL recommendation. Data rates specified to date include: up to 26 Mbit/s downstream from a central office or optical network unit to a remote terminal; 4.8 Mbit/s upstream for asymmetric transmission; and, up to 14 Mbit/s for symmetric transmission. For VDSL systems, a frequency-division duplexing (FDD) technique has been adopted, whereby separate frequency bands are allocated to upstream and downstream transmissions. Near-end cross talk (NEXT) is avoided by this duplexing technique. The predominant source of interference is therefore far-end crosstalk (FEXT).
Decision Feedback Equalizer (DFE) structures with cross-coupled filters have previously been considered for interference suppression in wireless CDMA communications [1] and fast Ethernet transmission [2].
Upstream power back-off methods are conventionally employed to allow remote users in a VDSL system to achieve a fair distribution of the available capacity in the presence of FEXT [3]. The upstream VDSL transmission rates which are achievable with the power back-off methods that have been contemplated to date usually depend on parameters such as a reference length, or the integral of the logarithm of the received signal power spectral density (PSD). The values assigned to these parameters are obtained by various tradeoffs between services to be offered, and allowed maximum line length. Consequently, the application of currently conventional power back-off methods result in a limited allocation of signal power for upstream transmission.